1. Field of the Invention
The present application generally concerns the field of novel synthetic methods for producing metallic inorganic compounds and more specifically concerns synthesis of lithium containing materials suitable for use in rechargeable electric cells.
2. Description of Related Art
The current expansion in the widespread use of portable electronic devices ranging from dvd (digital video disk) players and cellular telephones to PDAs (personal digital assistants) and laptop computers has greatly increased the demand for a rechargeable battery with high energy storage density. Such portable electronic devices require batteries that are light in weight and yet store a large amount of energy so that the devices can operate for a long period between recharging. In addition, the ideal battery should be exhibit a short recharge cycle time and be capable of a large number of recharge cycles before energy density or other characteristics deteriorate. Although disposable batteries, such as alkaline power cells, can be used in many of these devices, convenience and disposal problems generally mitigate against throw away batteries.
At first blush it might seem that a quick look at chemical potential tables could solve the problem of selecting ideal anode and cathode materials for a chemical cell. After all, by comparing half-cell potentials of candidate materials with the density of the material, one should be able to select cathode and anode materials with favorable weight properties. Lithium, the lightest of the metals, has the highest specific energy capacity of the practical materials and is a natural choice for use in chemical cells. However, simple half-cell comparisons reveal the electrical potential of a combination but says nothing about the overall properties of a cell. Even when a combination with high energy storage is developed, that combination may produce a cell that can be effectively recharged only a few times. The electrodes may deform or otherwise change, e.g., by dendrite formation, during the charging and discharging cycle. The chemical reactions may not be truly reversible resulting in loss of potential or energy storage with each recharge cycle.
Consequently, considerable research has been devoted to the design of advanced high capacity rechargeable battery systems. Currently many of the most promising systems under development use an alkali metal as the negative electrode (anode). Alkali metals combine high standard electrode potential with high energy capacity and high reactivity. This high reactivity results in nonstandard (as compared to traditional chemical cells) configurations. In particular, the standard aqueous electrolytes are replaced with solid or liquid nonaqueous materials. This may results in "different" electrolytes such as molten lithium chloride, lithium bromine and potassium bromide. Such high temperature electrolytes are not suitable for consumer electronics.
However, nonaqueous lithium ion batteries do appear to have the required characteristics for consumer use. These batteries are based on a lithium-ion intercalation anode, a lithium ion containing electrolyte, and a lithium-ion intercalation cathode. It turns out that lithium-ion accepting materials are not always easy to produce. These materials should be open-structured so as to be capable of readily accepting (during discharge) and releasing (during recharge) lithium ions. This generally means that the cathode materials must have a crystal lattice into which or out of which the lithium ions can readily move. Further, the materials typically have a layered or porous structure so that the ions can easily and rapidly move in and out. Further, these materials must maintain this structure through many charging and discharging cycles. It is likely that efflux and influx of ions will result in dimensional changes, but such changes are kept to a minimum in ideal materials.
The most studied cathode materials have been dichalcogenides of transition metals (e.g. titanium sulfide (TiS.sub.2)) or transition metal oxides (e.g. vanadium oxide (V.sub.6 O.sub.13)). These materials can form lithium-intercalated compounds that are capable of rapid and highly reversible chemical reactions. However, they must first be lithiated before use in a lithium ion battery system. In actual practice only a limited number of these compounds have thus far proven useful for lithium ion systems. In fact, only three types of materials are in common use, lithium manganese oxide (LiMn.sub.2 O.sub.4), lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), and mixtures thereof. These compounds are limited to an energy storage density of between 100 and 180 mAh/g (about 280-504 mWh/g). It is an object of the present invention to provide simple and reproducible methods of producing other lithium-containing cathode materials that have a higher energy density than the above mentioned lithiated cathode materials.